Make a hole in the ground of a video which is camera-tracked?
Firstly, you need to create the shape of the inside of the hole and make sure the object is on the foreground layer.

After that, you need to crop the generated Ground object to remove an area which matches the opening of the hole.

Here is the result (not realistic, just to show that it works):



This is the way I did it. I guess this might not be the best way to do it, but it works.

I am not familiar with Blender terminology. If someone is willing to post a more detailed or better answer, I would be glad to accept it.